Priority is claimed with respect to German Application No. 199 28 139.4-12 filed in Germany on Jun. 19, 1999, the disclosure of which is incorporated herein by reference.
The invention relates to a method and a device for lubricating a machine, especially an internal combustion engine.
U.S. Pat. No. 5,465,810 discloses an oil circuit with two different grades of base oil of different viscosities contained therein. Above a temperature limit, the two grades of oil can be mixed with each other to form an oil of the desired viscosity, which below (this temperature limit can be fractionated again into the two original grades of oil.
This prior art provides two separate oil reservoirs, each for receiving a grade of base oil, from which oil is taken, mixed temperature-dependently in the desired ratio and fed to the lubricating point. The oil flowing away from the lubricating point is collected, cooled in a heat exchanger and passed into a fractionating apparatus, in which the oil mixture is fractionated into its two grades of base oil. The two grades of base oil recovered in this way are fed to the respective oil reservoirs.
The disadvantage of this solution is the high expenditure for the arrangement of a separate fractionating apparatus and the necessity for the latter to be operated continuously.
As an alternative to this, this prior art publication describes an arrangement in which only one oil reservoir is provided, for simultaneously receiving two different grades of base oil of different viscosities and different relative densities. Above a temperature limit, the two grades of base oil can be mixed at least partially to form an oil mixture. Below a temperature limit, the oil mixture segregates and forms a two-layer oil reserve with a lower layer of oil of high relative density and high viscosity and an upper layer of oil of low relative density and low viscosity. Oil is taken from the upper region of the oil reserve for supplying the lubricating point. At lower temperatures, this oil is segregated oil of lower viscosity and at higher temperatures it is an oil mixture of the two grades of base oil of higher viscosity.
The disadvantage of this solution is the poor definiteness of the oil viscosity during warm operation, since in a transitional temperature range there is no complete or precise, definable mixing of the two grades of base oil. Furthermore, with simple means, for example in the form of a dipstick, only the total oil level, but not the proportionate amount of the two individual grades of base oil, can be determined.
The invention is based on the object of providing a method and a device for lubricating a machine, especially an internal combustion engine, by which the lubrication is adapted better to the operating state and the oil change intervals can be extended considerably.
By the method according to the invention, the machine in the cold state is lubricated with low-viscosity oil, for reducing friction and improved starting performance. For this purpose, an oil pump pumps a first oil of low viscosity in an oil circuit from a first oil tank to the lubricating points. The low-viscosity oil dripping from the lubricating points is collected in an oil sump and returned to the first oil tank. During this process, a second oil of higher viscosity is kept ready in a second oil tank. The first oil and the second oil have properties of such a kind that there is a limit temperature above which the first oil can mix with the second oil to form an oil mixture of average viscosity and, below the limit temperature, the oil mixture thus produced can be fractionated into the first oil and the second oil.
For suitable lubrication in the warm state, a machine needs an oil of higher viscosity than in the cold state. Therefore, according to the invention, in a way dependent on the warming up of the machine, when a predetermined switching temperature lying above the limit temperature is reached, the oil circuit is switched over by a first switching device in such a way that the second oil is fed from the second oil tank to the lubricating points, mixing with the residue of the first oil remaining in the machine to form an oil mixture of a higher viscosity than that of the first oil, the oil mixture is collected in the oil sump and is returned to the second oil tank, so that after a transitional time the second oil is replaced in the second oil tank by the oil mixture, which from then on is fed to the lubricating points.
After the machine has been switched off and cooled down, oil of lower viscosity is required again for lubricating the lubricating points for re-starting. Therefore, in the switched-off state of the machine, after cooling below the limit temperature the oil mixture in the second oil tank is fractionated into the first oil and the second oil. In an advantageous version of the method, this fractionation takes place automatically by using a first oil with a lower relative density than that of the second oil, so that an upper layer with the first oil and a lower layer with the second oil forms in the second oil tank. The first oil recovered in this way and located in the second oil tank is returned to the first oil tank, in that a connecting line between the second oil tank and the first oil tank is opened by means of a second switching device. In this case, the second oil remains in the second oil tank.
An advantageous version of the method consists in that the first oil, located in the second oil tank after fractionation has taken place, is returned to the first oil tank by means of a pressure gradient produced by a geodetic difference in height.
The oil circuit is preferably operated using two oil pumps, in that the oil or oil mixture is fed to the lubricating points of the machine by means of a first oil pump, and that the oil or oil mixture collected in the oil sump is returned to one of the two oil tanks by means of a second oil pump.
For precisely controlling the removal and return of oil or oil mixture from or to one of the two oil tanks, the removal of oil or oil mixture from one of the two oil tanks is advantageously controlled by means of a first switching device and the return of the oil or oil mixture to one of the two oil tanks is advantageously controlled by means of a third switching device. The switching devices may be controlled time-,viscosity- or oil-pressure-dependently and preferably temperature-dependently. The switching over of the first and third switching devices may take place independently of each other, but advantageously simultaneously.
In the device according to the invention, at least one switching device, preferably all the switching devices, is/are designed as thermostatic switches for the sake of a simple structural design.
Furthermore, for extending the oil change intervals, a condensate trap is provided in the oil line between the oil sump and the third switching device, as well as preferably a main-flow oil filter and optionally a secondary-flow oil filter.
To avoid overheating of the oil circuit in warm operation, as a consequence of which the lubrication deteriorates and the oil ages more rapidly due to decomposition processes, in an advantageous design of the device a heat exchanger is arranged between the third switching device and the second oil tank.
To achieve an adjusted oil mixture of the first oil and the second oil that is as precise as possible, it is desirable for as little of the first oil of lower viscosity as possible to remain in the machine after the cold-running phase. The oil sump is therefore preferably designed as a dry sump.